The present invention relates to fibre optic data links and in particular to the merging of multiple separate input data streams into a single data stream for transmission. More particularly, the present invention relates to the modulation in the optical domain of a signal for transmission.
Prior art methods of data transmission require multiple wire, high cost cables and suffer from limited transfer distance, degraded Front of screen (FOS) performance and Electro-Magnetic Compatibility (EMC) problems. The majority of computer systems at present utilise CRT monitor display systems and it is the analogue nature of the signals required by these display monitors which is responsible for imposing the distance limits over which the data can be delivered. As LCD flat panel monitors become more prevalent, due to decreasing costs, it is no longer required to deliver the data in an analogue format but to use their native digital format for transmission from the PC system unit to the display. Until recently this has been achieved by a double conversion process, firstly digital-analogue and secondly analogue to digital, which allowed the industry standard analogue interface to be used but suffers the problems referred to above and also further signal degradation from the double conversion process.
Fibre optic data links are well known and have the advantages of good noise immunity and high bandwidth. The current technology of fibre optic data links is generally designed for telecommunications applications in which communications over distances of tens of kilometres is required with a very low error rate. Such links are asynchronous digital links having multiple input data streams, and include, for example, ISDN. The data structures in the fibre optic link are very different to that used by the equipment between which communication is taking place by means of the fibre optic link.
Whilst such known fibre optic links work well for telecommunications applications at, for example, 1.0 Gigabits/sec or at 2.4 Gigabits/sec, the cost of the link is high. In telecommunications applications, this cost is shared by the multiple separate pieces of equipment which are using the fibre optic link to communicate.
The benefits of good noise immunity and high bandwidth mean that the use of fibre optic links for non-telecommunications applications is increasing. Such applications are distinguished from telecommunications applications by virtue of the fact that they rarely exceed 150 metres in length and are frequently as short as 2 metres in length. The cost of a telecommunications type of fibre optic link for such an application is between 10 and 100 times too expensive. The physical size of the equipment for a telecommunications fibre optic data link is too large for easy incorporation into a personal computer, computer display or an input/output sensor. When used as a data link from a personal computer to a computer display, the video data that is sent from the personal computer to the computer display can be permitted to have transmission errors, but the synchronisation (or control) signals cannot be permitted to have transmission errors, otherwise the displayed image will break up and the errors will be visible to the end user.
GB Patent Application No. 2339653, discloses an isochronous output data stream format in which a single n bit word is transmitted as m multiple synchronous data streams where the length of the word transmitted in each data stream is n/m, the output data stream containing multiple isochronous different data rate data streams and one or more asynchronous data stream or streams. One bit from each of the m multiple synchronous data streams are combined and transmitted as a single signal at one of 2 m analog levels.
Generally the prior art systems for transmitting data in fibre optic systems have problems associated with them including the use of purely binary systems for optical communications. Specifically, the process of combining analog signals in an electric domain has many unwanted effects associated with it, such as jitter and overshoots.
Thus, there is a need for an alternative method to using binary systems for multilevel optical communication, whereby the method is preferably cost-effective.
Accordingly the present invention provides an optical amplitude modulator for modulating signals for transmission by a fibre optic link comprising a plurality of optical transmitters, each of said transmitters being driven by an electronic circuit, wherein each of said optical transmitters provides a light output, and said light outputs of said optical transmitters are combined optically for transmission as an optical output signal.
In a preferred embodiment, each of the plurality of optical transmitters has an associated optical filter, each of said filters having a different level of opaqueness, for filtering light output from the optical transmitter. Since the plurality of optical transmitters are identical to each other, the system is advantageous in that it is simple to manufacture identical transmitters.
In another preferred embodiment, each of the plurality of optical transmitters has a different level of semiconductor diffusion, causing the light output from each of the plurality of optical transmitters to differ, according to the level of diffusion. Since filters are not required in this embodiment, the system is more compact and the costs associated with filters are also removed.